So-called cellular screening methods are often used to determine pharmaceutical effects of test substances. In these, living cells are brought in contact with the test substances in order to determine the function of the latter as possible effectors on target components in a cell, for example receptors and/or ion channels. To this end, International Patent Application PCT/EP01/03209 proposes the use of a system in which the effect of a test substance on the activity of a cell is measured by a field-effect transistor placed underneath it.